The present invention relates to diffraction optics screen apparatus, and more particularly to an improved optical apparatus which provides the capabilities of full-color viewing, suppression of zero-order light, and on-axis viewing.
Diffraction optics diffusion screens employing holographic elements are well known in the art. U.S. Pat. No. 4,372,639 discloses a diffraction optics directional diffusing screen, for playback of light from a monochromatic light source. This type of screen employs a diffusion hologram, and exhibits a small and well-defined exit pupil, high and uniform gain across the exit pupil, and low backscatter. The light which passes through the holographic optical element without diffraction, known as zero-order light, illuminates the space in which the viewer is located to increase the ambient light level, which in many applications degrades the quality of viewing. Also, the viewing is off-axis, which may be undesirable for certain applications. Further, the simple diffusion hologram does not play back well in full-color. The white light incident on the hologram would be dispersed, i.e., diffracted at different angles for different wavelengths, creating a rainbow-like fan of colored light exiting each point on the hologram. Because the divergence of the projected beam onto the screen creates a wide variation in the incident angle, the resultant dispersion is so great that only a small overlap region between the red, green, and blue exit pupils is available for full-color viewing.
The problem of the zero-order light can be addressed by the addition of a fiberoptic faceplate, as shown in U.S. Pat. No. 4,586,781, assigned to the same assignee as the present application. The faceplate absorbs the zero-order light while transmitting the diffracted light, and scrambles the multicolored diffuse cones of light about its fiber axis to create a uniform full-color exit cone. The particular arrangement shown in U.S. Pat. No. 4,586,781 also provides off-axis viewing. The optical fibers of a conventional fiber optic faceplate are all oriented parallel to each other, so that the exiting diffuse cones of light are all parallel to one another. To obtain any appreciable overlapped full-color viewing region requires making the diffuse cones very large. The result is that much of the light is wasted and the main advantage of the holographic screen, its high gain, is severely compromised. One could use a lens on the front of the screen to achieve the focusing of the diffuse cones so as to preserve the high screen gain. However, a convex surface facing outward in a high ambient level environment is unacceptable, because it will reflect ambient light or sunlight from a wide range of positions directly into the viewer's exit pupil.
U.S. Pat. No. 4,586,780, also assigned to the same assignee as the present application, discloses a directional diffusing screen with suppressed zero-order light. The entire disclosure of U.S. Pat. No. 4,586,780 is incorporated herein by this reference. The optical apparatus described in this patent comprises a laminate of a transmission hologram, a fiber optic faceplate, and a holographic screen. Light incident on the transmission hologram is diffracted off-axis. The fiber optic faceplate is designed so that the diffracted light is parallel to the fiber axes and passes directly through the fibers without being scrambled about the fiber axis. However, the zero-order light undiffracted by the transmission hologram is absorbed by the faceplate. Light diffracted by the transmission hologram and passing through the faceplate are incident on the diffusion hologram and are diffracted back on-axis into a diffused exit pupil. This screen possesses many attractive features, including those of a simple diffraction optics diffusing screen, small and well-defined exit pupil, high and uniform gain, and low backscatter. In addition, it suppresses the zero-order light, blocks the ambient light from entering the display interior, and allows on-axis viewing (i.e., where the projection axis and viewing axis lie normal to the screen). However, one drawback of this optical apparatus is that it cannot be played back in full-color but requires near-monochromatic light. If one attempted to play back the screen in full color, white light incident on the transmission hologram would be dispersed, i.e., diffracted at different angles for different wavelengths, creating a rainbow-like fan of rays exiting each point on the transmission hologram. The rays from the transmission hologram would enter each optical fiber of the faceplate as a converging fan of rays. While passing through the optical fiber, they would be scrambled about the central axis of the fiber, so that they would exit the fiber as a circularly symmetric diverging cone of light. Since each optical fiber will emit a cone of light, and there are millions of optical fibers in the faceplate, the effect of the faceplate is to create a diffuse exit beam. This diffuse exit beam is incident on the diffusion hologram, which is only able to efficiently diffract non-diffuse light. The result is the diffusion hologram diffracts the input light only weakly, the resultant exit pupil is color blurred and larger than desired, and a substantial portion of the input light passes through the diffusion hologram undiffracted. The ultimate result is a dim, color-nonuniform screen.
It is therefore an object of this invention to provide a screen possessing the advantages of the directional diffraction optics diffusing screen, and which is in addition able to block the zero-order beam and allow on-axis full-color viewing.